Abstract/Summary

The Over Water SSSI was first notified because of the biodiversity of the aquatic macrophytes and marginal vegetation at the site, as well as the comparative eutrophy of the lake compared to surrounding water bodies.
Recent aquatic macrophyte surveys suggest that the site has deteriorated, and has lost “characteristic” species for which the site was originally notified. Reduced water levels and nutrient enrichment have been suggested as causes of change.
Over the study period there is no evidence of a trend in water levels or in the volume of water abstracted from Over Water. The length and severity of drawdown were greatest in 1995, 1996 and 2003 and abstraction volume was also high.
In “dry” years, abstraction could potentially account for much of the variation in water level. There is considerable variation in water level that cannot be explained by abstraction. This indicates that there are unquantified losses of water from the lake and/or errors associated with the known components of the water balance.
The present evidence suggests that high water level fluctuation, especially in dry years with high abstraction, could have an adverse effect on the macrophyte community. However, it is also possible that the presence of Elodea nuttallii may be responsible for the loss of native deeper-water species.
Water chemistry data from the Environment Agency for the period April 1985 - December 1989 were analysed. These data suggest that Over Water had a moderate alkalinity and productivity in the late 1980s. The concentrations and seasonal patterns are what would be expected for this lake type apart from the moderately high concentrations of orthophosphate throughout the year. However, simple modelling suggests that phosphorus limits phytoplankton growth in the lake.
Estimates of compliance with the Water Framework Directive, based on current understanding of the procedures, suggests that the average phytoplankton chlorophyll a concentration would give a ‘moderate’ status for Over Water.
Wildfowl appear to have a modest effect on the nutrient load and phytoplankton response. Estimates suggest they are responsible for about 5% of the TP load, 9% of the SRP load and an increase in average chlorophyll a by about 1.3 mg m-3.
Current hydrological and water quality data are needed to establish the current ecological status of the site and to address the issue of recent ecological change. Then it will be possible to evaluate the likely effects of water level change and eutrophication on the ecology of the lake.